This invention relates to electron multipliers, and more particularly, to an improved multiplier and methods and apparatus for rounding off the sharp circular edges, fissures and protrusions at the ends of the holes in the dielectric plate of a channel-type electron multiplier.
In the prior art, it has been the practice to construct an electron multiplier of a glass plate which is more or less solid except for cylindrical holes having parallel axes, the holes extending completely through the plate. For example, see U.S. Pat. No. 3,449,582. When the plate, sometimes called a microchannel plate (MCP), is hydrogen fired, the hole surfaces become secondary emissive. However, the plate usually has two conductive faces which lie in two parallel planes, respectively, both plates lying, in turn, perpendicular to or lying a few degrees from ninety from the axes of the holes. Thus a cylinder of a hole diameter and located concentric with the axis of a hole intersects both of said planes on an approximately circular line where sharp, approximately circular edges exist. These sharp edges then exist at each end of each hole. Fissures and protrusions also exist at and near these sharp edges.
In accordance with the present invention, it has been discovered that the existence of the said sharp circular edges and the fissures and protrusions have several very serious disadvantages.
In the first place, the sharp edges and protrusions create enormous electric field intensities which causes field emission. The electron field emission on the input ends of the holes then acts as a source of undesired primary electrons which are multiplied just the same as the desired primary electrons. The undesired primaries and the undesired secondaries which are generated as a result of the undesired primaries then add substantially to the tube noise.
Still further, it is common to use a photocathode as a source of desired primary electrons. In such a case, with the very high electric field intensities created by the said sharp edges, desorption of positive ions adsorbed by the plate can take place. Since the photocathode is conventionally maintained at a potential negative with respect to the plate, some, if not all, of the desorbed positive ions will invariably bombard the photocathode and contaminate it, i.e. eventually cause its efficiency to be reduced so much that its useful life is considerably shortened.